1) You need a newer version of UOAM than that supplied for Valhalla - one that supports multiple maps (like Tokuno, Malas, Ilshenar etc)
2) The basic game comes with a default map file of map0.mul
3) Valhalla has its own map file and this is mam0.mul
4) Rename map0.mul to map0_old.mul (just in case you need it back)
5) Right Click mam0.mul (the Valhalla map file) and select copy
6) Right click in the widow (not on a file name) and select paste
7) You should now have a file called mam0 - Copy.mul (just above the file mam0.mul) Rename mam0 - Copy.mul to map0.mul
9) When you run UOAM (depending upon operating system and security) you will probably need to run it as Administrator (right click and select Run as Administrator or set it up in the shortcut properties tab)
10 When you run UOAM the Lost realms should be the first 2 maps